The use of optical fibers for the transmission of information via impulses of light has grown steadily in recent years as an alternative to conventional wire links, i.e. metal conductors. Because optical fibers have a greater bandwidth and lower impedance than metal conductors, the fibers can carry comparatively more information per unit time over comparatively greater distances. In addition, optical fibers are considerably less susceptible than metal conductors to unauthorized "taps", and eliminate R.F. problems and the need for electrical isolation interfaces such as opto-isolators
While optical fibers are superior in many respects to wire links for information transfer, it is not economical to use the fibers as a direct replacement for individual metal conductors. Optical fibers can be, however, an economical alternative to wire links where their superior bandwidth may be taken advantage of via multiplexing such that numerous conventional wire links may be replaced with a single fiber. Or, where optical fibers are desired for transmission security reasons alone regardless of cost, they can at least be made more economical if they can be used to replace several wire links. For example, computer equipment is commonly connected via RS232 or RS422 interfaces, with the interface cables running between equipment in different locations in a building or between equipment in different buildings. Such cables are susceptible to unauthorized taps via which proprietary data may be misappropriated, or via which unauthorized and potentially disastrous entry to a computer system may be obtained. Accordingly, where security is a concern it is highly desirable to substitute more secure optical fibers for the various lines of the cables. At the same time, however, it is desirable to maintain the architecture and protocol of the interface or, in other words, provide that the optical fiber link be function-wise "transparent", such that modifications to equipment may be avoided. While this may be accomplished by merely substituting each conductor of a cable with an optical fiber, and providing fiber optic couplers on either end of each fiber to translate back and forth between electrical and light signal transmissions, it is a waste of the superior bandwidth of the fibers, which have the capacity to carry (at the typical communications frequencies of MIL 188C, RS232 or RS422 interfaces) several lines worth of information. The more economical alternative is of course to multiplex several lines over a single fiber.
In most typical data communication multiplexers, each synchronous/asynchronous serial channel is interfaced via a USART (Universal Synchronous/Asynchronous Receiver-Transmitter). In the case of asynchronous mode, each USART must be "programmed" for baud rate, number of bits per byte, number of stop bits, etc. These multiplexers are not transparent in that channel interfaces cannot be mixed without reprogramming. In addition, they do not encode any control signals, i.e. Request-to-send, Clear-to Send, Carrier Detect, Data Terminal Ready, Data Set Ready, etc . . . This can be a major drawback in some systems such as IBM.RTM. Bi-Synch where the system software relies on the carrier detect signal to determine when to transmit (half-duplex system). Also, typical multiplexers are designed in respect of a known predetermined timing relationship between the signal to be multiplexed, keying on one or more control signals which are indicative of the timing of the data signals. However, where it is not known for the purposes of design what the timing relationship between signals is to be, as for example may be the case for security reasons, typical multiplexing circuits are of no use.
Accordingly, the present invention provides a fiber optic multiplexing circuit in which signals are multiplexed over an optical fiber without regard to timing relationships or signal functions (i.e. control vs. data). The multiplexing circuit of the present invention is also transparent in that it requires no programming, such that it may be introduced as a substitute for several existing interface lines without affecting signal timing on either end of the interface.